The invention relates to increasing the gap between a lead frame and a semiconductor die.
The use of "leads over chip" (LOC) semiconductor die assemblies has become relatively common in the semiconductor industry. Referring to FIGS. 1A and 1B, a packaged LOC assembly 60 includes lead fingers 112 extending over portions of the active surface 116 of a die 102. The die surface 116 is adhered to the lead fingers 112 using adhesive layers 114, such as tape, screen print, or spin-on adhesive dielectric layers disposed between the underside of the lead fingers and the die surface. Adhesive dielectric layers can be made of a polyimide film or an adhesive tape such as KAPTON.TM. tape, a trademark of DuPont.
Bond wires 106 (typically gold, although aluminum and other metal alloys have been used) extend between corresponding bond sites 108 on lead ends 122 and bond pads 110 on the active die surface 116. Alternatively, the conductive connections between the die 102 and the lead extensions 112 may be made by tape automated bonding (TAB), in which the lead ends are bonded directly to bond pads 110 by methods known in the art.
The most common manner of forming a plastic package around a die assembly is by molding, more specifically transfer molding. In this process, a semiconductor die is suspended by its active surface from the underside of lead fingers of a lead frame (typically copper or an alloy) by an adhesive layer, as illustrated in FIGS. 1A and 1B. After the bond pads of the die and the lead ends of the frame are electrically connected by bond wires, the resulting LOC die assembly is placed in a mold cavity and encapsulated in a thermal setting polymer which, when heated, reacts to form the final packaging material 134, which is a highly crosslinked matrix no longer capable of being re-melted. The packaged die assembly may be a dual-in-line package (DIP), zigzag-in-line package (ZIP), small outline J-lead package (SOJ), quad flat-pack (QFP), plastic leaded chip carrier (PLCC), surface mount device (SMD), or other plastic package configuration.
The thermal setting polymer generally includes three major components: an epoxy resin, a hardener (including accelerators), and a filler material. Filler materials usually include a form of fused silica, although other materials such as calcium carbonates, calcium silicates, talc, mica, and clays have also been used. Powdered fused quartz is an example of a filler used in encapsulants. The filler materials are relatively hard, particularly when compared to the die surface 116.
Fillers are used to reinforce the polymer to provide additional package strength, to enhance thermal conductivity of the package, to provide enhanced resistance to thermal shock, to reduce the coefficient of thermal expansion of the composite material, and to reduce the cost of the encapsulating material as compared to unfilled polymer.
The filler material includes small filler particles 130 (typically between 75 and 125 micrometers or .mu.m). Some of the filler particles 130 can be trapped in a gap 126 (FIG. 1B) between the underside of the lead finger 112 and the die surface 116. The trapped hard filler particles may place the active die surface 116 under residual stress at the points of contact of the particles. The particles may then damage the die surface 116 or conductive elements thereon when the package is further stressed (mechanically, thermally, or electrically) during post-capsulation handling and testing.
One method of reducing stress to the die surface due to trapped mold compound particles is described in U.S. patent application Ser. No. 08/857,200 entitled "Compression Layer on the Leadframe to Reduce Stress Defects," filed May 15, 1997, and having the same assignee as the present application. As shown in FIG. 2, a packaged LOC assembly includes a stress relief layer 140 that is attached to the under surface 128 of the lead finger 112. The material of the stress relief layer 140 may be a soft metal such as silver or other relatively inert metal, or an alloy thereof having generally low hardness, including pure metals such as palladium and platinum and their alloys. The thickness of the stress relief layer 140 is generally about 1 to 5 .mu.m (or about 39 to 197 microinches), which is sufficient to allow relatively deep penetration by filler particles 130. Alternatively, the soft material of the stress relief layer 140 may be a polymer designed to have a low hardness, including epoxies, polyimides, acrylics, and silicones. The stress relief layer 140 extends from the outer edge 138 of the adhesive layer 114 to the outer edge 136 of the die 102 or beyond, such that the outer edge 124 of the stress relief layer 140 extends to or past the edge 136 of the die 102. Damage to the die surface 116 is reduced since any trapped particles 130 will preferentially penetrate the soft material layer 140.
Another method of reducing stress to the die surface due to trapped particles is described in U.S. patent application Ser. No. 08/614,618, entitled "Stress Reduction Feature for LOC Lead Frame," filed Mar. 13, 1996 and also having the same assignee as the present application. As shown in FIG. 3, the packaged LOC assembly includes a slot or recess 113 that is formed in the lead finger 112 by etching, machining, eroding, removing material with an electron beam, or by other processes known in the art to reduce the thickness of the lead finger 112 proximate the active die surface 116 between the portion of the lead end 122 attached to the adhesive 114 and the outer edge 136 of the die 102. The recess 113 creates an enlarged space 117 between the active surface 116 and the lead finger 112, thus reducing the likelihood that filler particles 130 can be trapped between the lead finger 112 and the active die surface 116.